1. Field of the Invention
The present invention relates to an optical element having an antireflection action corresponding to an incidence angle of light beams and a scanning optical system having the optical element. In addition, the present invention is preferable for image forming apparatuses such as a laser beam printer, a digital copying machine and a multi-function printer that have an electrophotographic process, which deflects light beams emitted from light source means by a light deflector (deflection means) and optically scans a surface to be scanned via scanning optical means including an optical element that has an fθ characteristic and is provided with a microstructure grating to thereby record image information.
2. Description of the Related Art
In a conventional scanning optical system such as a laser beam printer (LBP), light beams emitted from light source means, which are optically modulated according to an image signal, are periodically deflected by a light deflector consisting of, for example, a polygon mirror and are focused in a spot shape on a surface of a photosensitive recording medium to optically scan the surface of the recording medium by scanning optical means having an fθ characteristic and thereby perform image recording.
FIG. 11 is a main part sectional view in a main scanning direction of the conventional scanning optical system (main scanning sectional view).
In the figure, reference numeral 91 denotes light source means, which consists of, for example, a semiconductor laser. Reference numeral 92 denotes a collimator lens, which converts divergent light beams emitted from the light source means 91 into substantially parallel light beams. Reference numeral 93 denotes an aperture stop, which limits light beams that pass therethrough to shape their beam shapes. Reference numeral 94 denotes a cylindrical lens, which has a predetermined power only in a sub-scanning direction and focuses light beams that have passed the aperture stop 93 on a deflection surface (reflection surface) 95a of a light deflector 95 discussed below substantially as a linear image within a sub-scanning section.
Reference numeral 95 denotes a light deflector as deflection means, which consists of, for example, a polygon mirror (rotatable polygon mirror) of four-side structure and rotates at a constant speed in a direction of arrow A in the figure by driving means (not shown) such as a motor.
Reference numeral 96 denotes a scanning lens system functioning as scanning optical means having a condensing function and an fθ characteristic. The scanning lens system 96 consists of two scanning lenses of first and second scanning lenses 96a and 96b, focuses light beams based on image information reflected and deflected by the light deflector 95 on a photosensitive drum surface 97 functioning as a surface to be scanned, and establishes a conjugate relationship between the deflection surface 95a of the light deflector 95 and the photosensitive drum surface 97 within the sub-scanning section, thereby acquiring a toppling correction function.
In the figure, divergent light beams emitted from the semiconductor laser 91 are converted into substantially parallel light beams by the collimator lens 92, and light beams that pass through the aperture stop 93 are limited to have their beam shapes shaped by the aperture stop 93. Then, the substantially parallel light beams are entered into the cylindrical lens 94. The substantially parallel light beams on the main scanning section among those entered into the cylindrical lens 94 exit as they are. In addition, the substantially parallel light beams on the sub-scanning section converge and are focused substantially as a linear image (linear image which is longitudinal in the main scanning direction) on the deflection surface 95a of the light deflector 95. Then, light beams reflected and deflected on the deflection surface 95a of the light deflector 95 are focused in a spot shape on the photosensitive drum surface 97 via the first and second scanning lenses 96a and 96b and optically scan the photosensitive drum surface 97 at a uniform speed in a direction of arrow B (main scanning direction) by rotating the light deflector 95 in the direction of arrow A. Consequently, image recording is performed on the photosensitive drum surface 97 functioning as a recording medium.
However, the above-mentioned conventional scanning optical system has problems described below.
In recent years, it has become common to produce scanning optical means of a scanning optical system (scanning lens system) from plastics with which an aspherical surface shape is easily constituted and manufactured. However, in a plastic lens, it is difficult to apply antireflection coating on a surface of the lens due to technical reasons and reasons relating to costs. Thus, Fresnel reflection occurs on each lens surface.
FIG. 12 is an explanatory diagram showing angle dependency of P-polarized light reflectance and S-polarized light reflectance at the time when light beams are entered into, for example, a resin optical member with a refractive index of n=1.524. As shown in the diagram, Fresnel reflection on each optical surface (lens surface) ranges from several % to as large as 10% or more depending on an incidence angle.
Therefore, Fresnel reflection light generated on a lens surface without the antireflection coating is reflected on other lens surfaces and finally reaches a surface to be scanned to turn into ghost.
For example, as a first case, with respect to axial light beams, Fresnel reflection light is multi-reflected between any two surfaces among incidence surfaces and exit surfaces of the respective first and second scanning lenses 96a and 96b and reaches the surface to be scanned 97.
As a second case, when a lens surface 96a1 relatively close to the light deflector 95 of the first and second scanning lenses 96a and 96b has a recessed surface shape and incident light beams are nearly vertical as shown in FIG. 11, Fresnel reflection light on this lens surface 96a1 returns to the light deflector 95 and is reflected on the deflection surface (reflection surface) 95a of the light deflector 95 to pass through the scanning optical means 96. Thereafter, Fresnel reflection light reaches the surface to be scanned 97 to turn into ghost. When a quantity of ghost light exceeds approximately 1% of regular light beams, deterioration of an image becomes conspicuous, depending on an image forming system of a laser beam printer (LBP).
In addition, as a third case, surface reflection light reflected on the surface 97 of a photosensitive drum (photosensitive body) arranged in a position on the surface to be scanned may be reflected on any of incidence surfaces or exit surfaces of the respective first and second scanning lenses 96a and 96b and return to the photosensitive drum again to turn into flare light. A surface having a particularly large influence is an exit surface 96b2 of the second scanning lens 96b close to the surface to be scanned 97.
Up to now, in view of these cases, power distribution is adjusted to design a scanning optical system such that ghost light is not condensed on a surface to be scanned in order to reduce an influence of the ghost light. As a result, a degree of freedom of design is limited.
As another method, a method of optimizing an amount of transmitted light is proposed in, for example, Japanese Patent Application Laid-Open Nos. 2000-206445 and 2001-66531.
In Japanese Patent Application Laid-Open No. 2000-206445, it is attempted to solve the problem by appropriately setting diffraction efficiency of a diffraction grating surface provided in a scanning optical means. That is, desired power distribution is set for the purpose of correcting magnification chromatic aberration or focus to cut a grating with a desired pitch, and a height (depth) of a grating on the diffraction grating surface is appropriately set, whereby diffraction efficiency of diffraction light (primary diffraction light) to be used is changed from on-axis to off-axis and this change offsets a change in transmissivity generated on other deflection surfaces.
However, with this method, diffraction light of another order (also referred to as unnecessary diffraction light) increases when diffraction efficiency of diffraction light to be used is reduced. The increased diffraction light of another order reaches the surface to be scanned unless it is blocked by appropriately providing light shielding walls or the like, and turns into flare light, resulting in a factor of deterioration of an image.
Japanese Patent Application Laid-Open No. 2001-66531 discloses a condition for not allowing surface reflection light reflected on a surface of a photosensitive drum arranged in a position on a surface to be scanned to return to a scanning lens by working out a position of a return mirror and an incidence angle to a photosensitive drum. However, this also becomes a restriction for an arrangement of parts (optical elements) in terms of designing.